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Why does warm water freeze faster than cold? Researchers from Singapore believe they've cracked the long-standing paradox – it's down to the way stretched hydrogen bonds store energy.

The warm-to-frozen question has vexed boffins including Aristotle, Francis Bacon and Descartes, but didn't become part of accepted science until this publication, simply entitled Cool, was published in 1969 and hung the name “the Mpemba effect” on the phenomenon (after the child, Erasto Mpemba, who persuaded Dr Denis Osborne to check his observations.

The theory offered by the Singaporean researchers in this paper is that the Mpemba effect arises from the behaviour of stretched hydrogen bonds in water. Hydrogen forms covalent bonds with oxygen to make individual water molecules; and that these hydrogen atoms are also attracted to the oxygen atoms in neighbouring molecules, in a hydrogen bond.

These hydrogen bonds are stretched, because the individual molecules repel each other – and the warmer the water is, the greater the stretching (because the molecules are being forced apart by the heating). Stretched bonds store energy, and that energy is released when the water is cooled.

The last bit of their reasoning is simple: the relaxation time of those hydrogen bonds is proportional to the initial energy – and there's an exponential function at work in the release of that energy.

“Cooling the water as the source in a refrigerator as a drain, the H-O bond releases its energy at a rate that depends exponentially on the initially storage of energy”, they write.

The hypothesis should, The Register supposes, be relatively easy to test: different initial states could be calculated for different temperatures, and used to predict the degree of the Mpemba effect. ®